Among the many embodiments of such devices proposed to date, the most accurate are those incorporating two pluralities of conductive tracks, the tracks of each set being parallel and those of each plurality being perpendicular to those of the other plurality. These tracks are disposed face to face on an individual support for each plurality of tracks, one of these supports being a flexible membrane, and are maintained at a spacing from those of the other plurality by a number of spacing elements interspaced between the membranes. The tracks of one of the two pluralities are capable of being brought into momentary contact with the tracks of the other plurality by localised deformation of the flexible membrane in its portions located between these spacing elements.
Among the most recent documents dealing with a structure of this kind, produced with modern materials and by making use of an advanced technology, there may be mentioned, by way of example, European Patent Application No. 0,145,651.
In this document, as also in other previous documents such as Japanese Kokai 57-19476; 57-37909 and 57-37910, as well as the following patent specifications, U.S. Pat. No. 3,304,612, FR No. 2,453,452 and GB No. 2,046,450, the tracks of each of the two pluralities of tracks are attached electrically, at a corresponding end, to separate points of a collecting resistor specific to each plurality, the ohmic value of this resistor varying linearly along its entire length.
The electrical signal perceived by these collecting resistors when a pressure has been applied to a given point of the flexible membrane of the device which incorporates them, must generally be "processed" by an assembly of complex and delicate electronic and logic circuits, with the quality of the results dependent on, at the same time, both the manner of detection used and the structural characteristics of these circuits.
It is thus, for example in the case of the solution advocated in the document EP No. 0,145,651, that the evaluation of the signal received by the collecting resistors is carried out by a "flip-flop" type device, yielding first of all, information characteristic of the abscissa of the point looked for, then another signal relating to the ordinate of this point, and so on, this mode of operation resulting, for the user, in a perceptible limitation in the speed of repetitive operation of the device.
This time delay is due, to a large extent, to a capacitance effect intervening at the level of the conductive tracks as a result of the intermittent operation of the aforementioned flip-flop device.